U.S. Pat. No. 2,350,440 to John O. Almen discloses a device used for measuring the intensity of shot blast peening on metal parts, commonly known as an Almen gage. Almen gages measure the intensity of the peening process by measuring the curvature of thin metal test strips, after one side of the test strips has been peened. The unbalanced stress in the peened surface causes the test strips to bow. The Almen gage uses two spaced knife edged supports and a measurement indicator with an reciprocating feeler to measure the curvature of the test strip. The spaced knife edged supports hold the test strip as the feeler located between the knife edges engages the test strip. Gaging the height of the arc or bend of the test strip between the two predetermined contact points along the knife edged supports provides an indication of the intensity of the peening process.
Later refinements of the Almen gage have replaced the knife edged supports with four round contact balls with the operator manually holding the test strip in place for measurement. Another version of the Almen gage added a spring loaded finger to hold the test strip in place. The spring loaded finger was difficult to operate and subject to abuse and damage. Still another version of the Almen gage is disclosed in U.S. Pat. No. 5,297,418, in which magnets mounted in the base magnetize the contact balls, thereby holding test strips made of a magnetic material in place.
More recently, test strips have become available which are made out of a non-magnetic material, such as aluminum. Since these test strips cannot be held in place by magnets, the operator must either manually hold the test strip in place or a heavy block must be placed on top of the test strip to be measured. It is undesirable for the operator to hold the strip in place due to the variability of measurement results caused by variation in the holding force by the operator. It is also undesirable to use the heavy block, which normally has four balls or protrusions used to pinch or capture the strip measurements securely with the four balls provided on the gage. This method is adequate, but it is difficult to install the four balls in a flat plane needed to prevent strip deflection. Furthermore, even if the block has its four balls in an ideal flat condition, the balls on the gage may not be in a flat plane, and therefore unequal pinching force will result.